Automobile manufacturers have tried in recent years to reduce material costs, and one way of doing so is to use thinner gauge metal sheets for automotive body panels and other parts. While providing initial raw material cost savings, the thinner gauge metal presents some drawbacks. Thinner metals have less impact strength and also have lowered sound damping capacity. To overcome these drawbacks, automotive manufacturers have begun to apply sound damping, anti-flutter, and body panel reinforcement (BPR) coatings to the inside of body panels. Coatings on floorpans, firewalls, insides of door panels, and deck lids can be used to dampen or reduce road and engine noise, preventing sounds from traveling into the passenger compartment of the motor vehicle. Likewise, anti-flutter compositions are commonly used to prevent vibrations of doors and deck lids. They are usually extruded as beads or drops, often called “chocolate drops” in the industry, between reinforcing metal bars and the body panel. By varying the types and amounts of components, these compositions can provide different degrees of expansion and strength from the very soft to the very hard. The coatings that have been developed for the above purposes are sometimes solid laminate pre-cut sheets or pads that must be hand-applied to a substrate. Only small areas can be covered at one time, making the application of the layers time consuming and expensive.
Another drawback of commercial sheets and solid patch-type sound damping products is that the optimum performance is limited to narrow performance temperature ranges; their sound dissipation values drop off at the low and high end of an ambient temperature range. While newer, sprayable technologies reduce time and labor expenses, they likewise can perform well over only narrow temperature ranges.
Additionally, some of the sprayable products may deform the metal upon curing creating a visible defect on outer painted surfaces. In some cases it is visible only at cold temperatures. This phenomenon is often referred to as “print-through”, “read-through”, “telegraphing” and/or “ghosting”. It is desirable to eliminate such appearance defects.
Many sprayable sound-damping coatings must rely on the use of plasticizers to achieve a sprayable viscosity. Plasticizers can, and often do, present environmental concerns.
Accordingly, it would be desirable to provide a method of applying multiple coating compositions to substrates, to provide sound damping and reinforcement properties while minimizing print-through, without the drawbacks of the prior art.